1. Field of the Invention
This invention generally relates to Fourier filters, inspection systems, and systems for fabricating Fourier filters. Certain embodiments relate to a Fourier filter that includes an array of patterned features formed of one or more pigments on a substrate and configured to block light reflected and diffracted from structures on a specimen and to allow light scattered from defects on the specimen to pass through the substrate.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate such as a semiconductor wafer using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Many different types of inspection tools have been developed for the inspection of semiconductor wafers. Defect inspection is currently performed using techniques such as bright field (BF) imaging, dark field (DF) imaging, and scattering. The type of inspection tool that is used for inspecting semiconductor wafers may be selected based on, for example, characteristics of the defects of interest and characteristics of the wafers that will be inspected. For example, some inspection tools are designed to inspect unpatterned semiconductor wafers or patterned semiconductor wafers.
Inspection tools for unpatterned wafers are generally not capable of inspecting patterned wafers for a number of reasons. For example, many unpatterned wafer inspection tools are configured such that all of the light collected by a lens or another collector is directed to a single detector that generates a single output signal representative of all of the collected light. Therefore, light scattered from patterns or other features on the specimen will be combined with other scattered light. As such, light scattered from patterns or other features on the wafer cannot be detected separately from other scattered light thereby hindering, if not preventing, defect detection.
Patterned wafer inspection is of particular interest and importance to the semiconductor industry because processed semiconductor wafers usually have a pattern of features formed thereon. Although inspection of unpatterned wafers, or “monitor wafers,” which have been run through a process tool, may be used as a gauge for the number and types of defects that may be found on patterned wafers, or “product wafers,” defects detected on monitor wafers do not always accurately reflect the defects that are detected on patterned wafers after the same process in the same process tool. Inspection of patterned wafers is, therefore, important to accurately detect defects that may have been formed on the wafer during, or as a result of, processing. Therefore, inspecting patterned wafers or product wafers may provide more accurate monitoring and control of processes and process tools than inspection of monitor wafers.
Many inspection tools have been developed for patterned wafer inspection. Some patterned wafer inspection tools utilize spatial filters to separate light scattered from patterned features from other scattered light such that the other scattered light may be separately detected. Since the light scattered from patterned features depends on various characteristics of the patterned features such as lateral dimension and period, the design of the spatial filter also depends on such characteristics of the patterned features. As a result, the spatial filter must be designed based on known or determined characteristics of the patterned features and must vary as different patterned features are being inspected.
One type of spatial filter that may be used as described above is a Fourier filter. Fourier filters are relatively useful for filtering light from repetitive patterns such as memory arrays formed on a wafer. At least two previous methods have been used for Fourier filtering. One method is a mechanical method. This method utilizes mechanical rods or other mechanical devices to block the diffraction pattern generated by array structures so that the energy from the array region is removed from the optical path of the inspection system. Another method is a liquid crystal method. This method utilizes a one-dimensional or two-dimensional liquid crystal device to block the diffraction pattern generated by array structures so that the energy from the array region is removed from the optical path of the inspection system.
Although the above Fourier filtering methods have been relatively widely used, these methods do have a number of significant disadvantages. For example, the mechanical method has a number of disadvantages, particularly for flood illumination based systems. In particular, the diffraction peaks for flood illumination based systems are dots. Mechanical rods, therefore, block excessive amounts of light in such systems thereby reducing the overall defect signals. In addition, the mechanical method induces significantly more Fourier filter ringing that causes periphery energy leakage in output generated during inspection of the array region thereby reducing defect sensitivities in the array region. In particular, Fourier filters in the form of periodic blocking bars can diffract light into undesirable directions, which is commonly referred to as ringing or side lobes, thereby degrading the imaging quality. Therefore, the Fourier filter can produce significant distortion at the image plane, which adversely affects the ability of the inspection system to detect defects on the wafer with high accuracy. Furthermore, since the rods must have a relatively large diameter in order to be structurally sound, only a limited number of rods can be used. Otherwise, the entire plane would be blocked by the rods.
Currently used liquid crystal Fourier filter devices are programmable and capable of filtering out diffraction dots in a two-dimensional manner. However, there are several factors that make the liquid crystal devices inappropriate for flood illumination based systems. For example, the use of a liquid crystal device as a Fourier filter utilizes the principle of light scattering. For a flood illumination based system, the light scattering significantly alters the wavefront of the system thereby causing severe degradation to the image quality. In addition, most liquid crystal material has a damage threshold at a wavelength of approximately 300 nm thereby making liquid crystal devices less than ideal for use in deep ultraviolet (DUV) based systems (i.e., systems that operate at wavelength(s) less than about 300 nm).
Chrome masks may also be used as Fourier filters. Chrome masks generally include opaque chrome regions arranged in a pattern on a substantially transparent substrate. Therefore, chrome mask Fourier filters are similar in structure to binary masks used for patterning a resist in a lithography process. Chrome mask Fourier filters can also be fabricated using a process similar to a reticle manufacturing process. For example, a chrome mask Fourier filter may be fabricated by depositing chrome on the substrate, patterning the chrome by lithography and etch, and cleaning the substrate with the patterned chrome formed thereon. Some disadvantages of using chrome masks for Fourier filters are the relatively long turn around time and relatively high costs associated with fabricating such Fourier filters. In particular, since the chrome masks are generally made with lithography and cleaning steps, fabricating such Fourier filters is time consuming and expensive. In addition, a dedicated lithography and cleaning station in the semiconductor fabrication facility (“fab”) for fabricating Fourier filters is not practical. However, designing a mask and sending the design out for fabrication would cause several days of down time for the inspection system, which is also not practical.
Micro-electro-mechanical system (MEMS) devices may also be used for Fourier filtering. MEMS devices have been proposed for Fourier filtering, at least in part, due to their programmable features. However, there is some difficulty associated with using MEMS devices for Fourier filtering. For example, using MEMS devices for Fourier filtering may be difficult due to the fill factor of the MEMS devices, which needs to be more than about 99%, the surface roughness, which needs to be less than about 2 nm for reflection mode, the pixel edge diffraction of the MEMS devices, and the pixel stability and wave front error of the MEMS devices. Advances have been made to address the fill factor of MEMS devices (e.g., with pixels having sizes of, for example, about 20 μm×about 400 μm) and wave front error of the MEMS devices (e.g., using a corrector plate) as described in commonly assigned U.S. patent application Ser. No. 11/464,567 by Chen et al. filed Aug. 16, 2006, which is incorporated by reference as if fully set forth herein. However, the use of relatively long pixels may create new process issues for MEMS device manufacturing, and relatively large area blockage on the long pixel side to accommodate design tolerance may present a new difficulty associated with using MEMS devices as Fourier filters. Whether each of these risks can be mitigated remains to be seen.
Accordingly, it would be advantageous to develop a system configured to inspect a specimen that includes a two-dimensional Fourier filter suitable for flood illuminated DF wafer inspection systems, which does not block excessive amounts of light such that the overall defect signals are not reduced, does not induce significant Fourier filter ringing and periphery energy leakage, does not significantly alter the wavefront of the system, does not cause degradation of the image, is structurally sound, and is suitable for use at DUV and other wavelengths.